A conventional form of electronic organ tone generator utilizes twelve independently tunable master oscillators, one for each tone nomenclature of the musical scale, and employs a separate frequency divider chain synchronized from each of the master oscillators, to obtain ranges of related notes. This system has no automatic mechanism for maintaining the master oscillators relatively tuned, so that stable master oscillators must be employed, and retuning is required from time to time. In addition, the tone signals of any one nomenclature are phase and frequency locked, which is not true of pipe organ tones, whereby the latter cannot be accurately simulated in pitch.
Another type of commercial organ utilizes independent oscillators throughout for the tones of the organ. The tuning problem is thus exacerbated but phase and frequency locking are avoided.
The advent of rate scaling, which is fully explained in an article published in Electronic Design 3, Feb. 1, 1968, by Richard Phillips, provides an economical technique which can be employed to divide any number by any other number, and to multiply any number by any other number, including fractional numbers, as A/B, and to add or subtract any number to or from any other number. The rate scaling technique and other digital methods are applied in the present invention to derive tone frequencies closely approximating true scale frequencies, as required by an electronic organ, from a single master oscillator by wholly electronic techniques.
A number of diverse implementations of the basic techniques have been devised, each of which has its advantages and its disadvantages. It is desirable to employ a master frequency of under 2 mhz, and in fact as low as possible, in order that MOSFET technlogy may be employed. Frequencies having the precise ratios 3/2 or 4/3 must be avoided, or undesired beats may be heard. Frequency deviations from true tempered scale frequencies must be minimized or at least maintained within musically acceptable limits. This combination of requirements is difficult to meet, though any one singly may be easily met.
Since only one master oscillator is required to achieve and maintain correct relative tuning between notes, it becomes pratical, in the present system, by means of a single control, to tune the master oscillator to obtain key transpositions.
It is feasible to obtain the full tone range of an organ by deriving each note separately by rate scaling, or to obtain only master oscillation frequencies in this way. The first mentioned possibility has the advantage of providing unlocked tones, but there are other practical advantages in so proceeding in terms of grouping or locating tone sources so as to simplify or eliminate wiring.
One method of implementing the single master oscillator set of an organ system otherwise conventional, is to utilize twelve conventional dividers each related to the next by essentially the twelfth root of two. This is referred to as the integer system. Its inventive component resides in the selection of appropriate multipliers, all of the form 1/A.
A more subtle system is to rate scale the output of the master oscillator to obtain twelve frequencies at ratios related each to the next by the twelfth root of two to provide octave of tone signals. These frequencies can then be scaled down by identical conventional dividers. This system is referred to as the rate scaled system. The selection of suitable rate scaling divisors is important.
In a third system, sub-groups of fewer than twelve dividers are each related to the next by the twelfth root of two, and the master oscillator frequencies so derived are further divided to obtain new master oscillators for each of the sub-groups. This is referred to as the complex system.
Useful series of integer numbers available for implementing the present invention are selected from among the series 116-219, 239-451, 349-659, 543-1024 and 254-359, all inclusive. These sequences include close approximations to numbers related as the twelfth root of two. For example, high C in an organ is properly 8372.hz. The first series of numbers suggests 116 .times. 8372.=970,000.hz as a master oscillator. The number 970,000 can then be divided by 116 to obtain approximately 8372.
Other numbers in the series are then employed for other notes in the same way. If an octave of tone signals is to be derived in this way the relation 3/2 or 4/3 is found to occur and errors of as much as .+-. 2.7 cents. This is musically undesirable but usable.
The 239 series requires a master oscillator at 2,000,000 hz and contains two possible sequencies of twelve integers, properly related.